The present invention relates to a method of manufacturing a filter, the apparatus used in the method and the filter produced by the method. More particularly, the invention relates to a method of making a filter in which multiple layers of filtration material are sealed together at the edges of the filter and marginal pieces of the filtration material are simultaneously trimmed off at the edges.
Many filters are made with multiple layers of filtration material. Some of these are of an envelope type, where fluid flows into or out of the envelope through all sides of the envelope. Others are unidirectional, where the layers form a series of filters through which the fluid flows one after the other.
Most filters are constructed by forming the filtration material in large quantities in relatively flat sheets. Filters are then constructed by dividing the large sheets of filtration material into individual units, such as by cutting or stamping. In the case of multilayer filters, multiple individual units may be stacked together in layers to form a series of filters. Often the edges of the filter units will be sealed together to prevent fluid from bypassing one of the filters in the series. In the case of a filter made from an envelope, one piece of filtration material may be folded over in half (or folded in some other manner) and the non-folded sides sealed together around the periphery of the filter. In either situation, the size of the individual filter units is usually slightly larger than the size of the desired final filter component. This makes it easier to stack layers of filtration material together or fold an individual filter unit over without worrying about exactness. Marginal edges of the filtration material are then trimmed off, usually after a seal has been made at the periphery of the filter.
A typical sealing mechanism for many filtration materials is ultrasonic welding. In this process layers of filtration material are clamped between a welding horn and an anvil, often referred to as a weld nest. Ultrasonic energy is transmitted to the welding horn. Friction between the layers of filtration material induced by the movement caused by the welding horn relative to the anvil causes the filtration material to heat up and fuse together.
The dimension of the welding horn has some practical limitations. If the welding horn is too narrow, the horn will not have very long endurance, and will need to be replaced frequently. If the interface between the horn and the anvil is not completely covered by filtration material, direct contact between the horn and the anvil will result, also causing damage to the horn.
As a result, most filters made by ultrasonic welding have a weld area of a quarter of an inch or wider. Especially for small filters, this means that a large percentage of the filtration material is fused together. The fused material is unusable for filtration.
After the welding step, the marginal areas of the filter are usually trimmed off, either to make the filter more aesthetically pleasing or because of space limitations when the filter is placed inside of a cavity. Typically, the trimming step is performed using a stamping operation and a steel ruled die. This involves a separate manufacturing step and tool, adding to the manufacturing cost of the filter. Minimizing the amount of filtration material involved in the weld would be an advantage, as would simplification of the manufacturing process.